This invention in general relates to interferometrically measurements associated with a photolithographic stage.
Interferometry is a well established metrology used extensively in microfabrication processes to measure and control a host of critical dimensions. It is especially important in manufacturing semiconductors and the like where requirements for precision are one or more orders of magnitude better than critical dimensions of approximately 100 nm or below.
Integrated circuits made of semiconductor materials are constructed by successively depositing and patterning layers of different materials on a silicon wafer. The patterning process consists of a combination of exposure and development of photoresist followed by etching and doping of the underlying layers and deposition of another layer. Typically each wafer contains multiple copies of the same pattern called “fields” arrayed on the wafer in a nominally rectilinear distribution known as the “grid.” Often, but not always, each field corresponds to a single “chip.”
The exposure process includes projecting the image of the next layer pattern onto (and into) the photoresist that has been spun onto the wafer. For the integrated circuit to function properly each successive projected image must be accurately matched to the patterns already on the wafer. The process of determining the position, orientation, and distortion of the patterns already on the wafer, and then placing them in the correct relation to the projected image, is termed “alignment.” The actual outcome, i.e., how accurately each successive patterned layer is matched to the previous layers, is termed “overlay.”
In general, the alignment process requires both translational and rotational positioning of the wafer and/or the projected image as well as some distortion of the image to match the actual shape of the patterns already present. It is is important that the wafer and the image be positioned correctly to get one pattern on top of the other. Alignment may also address distortion of the image. Other effects, such as thermal and vibration, may require compensation as well.
Alignment is generally implemented as a two-step process; that is, a fine alignment step with an accuracy of nanometers or even subnanometers to tens of nanometers follows an initial coarse alignment step with an accuracy of microns, and alignment requires positioning the wafer in all six degrees of freedom: three translation and three rotation. It is common, however, to consider adjusting the wafer so that it lies in the projected image plane, i.e., leveling and focusing the wafer, which involves one translational degree of freedom (motion along the optic axis, the—axis or a parallel normal to the—wafer orientation) and two rotational degrees of freedom (orienting the plane of the wafer to be parallel to the projected image plane), to be considered separate from alignment. Thus, “alignment” typically refers to in-plane translation (two degrees of freedom) and rotation about the projection optic axis (one degree of freedom).
The reason for this separation in nomenclature is the difference in accuracy required. The accuracy required for in-plane translation and rotation generally needs to be on the order of nanometers to tens of nanometers or of the order of 1% of the minimum feature size or critical dimension (CD) to be printed on the wafer. Current state-of-the-art CD values are on the order of several hundred nanometers and thus the required alignment accuracy is less than 100 nm. On the other hand, the accuracy required for out-of-plane translation and rotation is related to the total usable depth of focus of the exposure tool, which is generally close to the CD value. Thus, out-of-plane focusing and leveling the wafer require less accuracy than in-plane alignment.
In-plane alignment typically involves the use multiple axes interferometer systems monitoring changes in the position of the wafer stage or reticle. For example, a suitable interferometry system includes that disclosed in U.S. Pat. No. 5,801,832 entitled “Method Of And Device For Repetitively Imaging A Mask Pattern On A Substrate Using Five Measuring Axes” by M. A. Van Den Brink, which involve distance measuring interferometers as high-stability plane mirror interferometers (HSPMI).
Distance measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from a reference object.
In many applications, the measurement and reference beams have orthogonal polarizations and different frequencies. The different frequencies can be produced, for example, by laser Zeeman splitting, by acousto-optical modulation, or internal to the laser using birefringent elements or the like. The orthogonal polarizations allow a polarizing beam-splitter to direct the measurement and reference beams to the measurement and reference objects, respectively, and combine the reflected measurement and reference beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that subsequently passes through a polarizer. The polarizer mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams.
A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a “heterodyne” signal having a beat frequency equal to the difference between the frequencies of the exit measurement and reference beams. If the lengths of the measurement and reference paths are changing relative to one another, e.g., by translating a stage that includes the measurement object, the measured beat frequency includes a Doppler shift equal to 2vnp/λ, where v is the relative speed of the measurement and reference objects, λ is the wavelength of the measurement and reference beams, n is the refractive index of the medium through which the light beams travel, e.g., air or vacuum, and p is the number of passes to the reference and measurement objects. Changes in the phase of the measured interference signal correspond to changes in the relative position of the measurement object, e.g., a change in phase of 2π corresponds substantially to a distance change L of λ/(2np). Distance 2L is a round-trip distance change or the change in distance to and from a stage that includes the measurement object. In other words, the phase Φ, ideally, is directly proportional to L, and can be expressed as Φ=2pkL cos2 θ, for a plane mirror interferometer, e.g., a high stability plane mirror interferometer, where
  k  =            2      ⁢      π      ⁢                          ⁢      n        λ  and θ is the orientation of the measurement object with respect to a nominal axis of the interferometer. This axis can be determined from the orientation of the measurement object where Φ is maximized. Where θ is small, Φ can be approximated by Φ=2pkL(1−θ2).
In some embodiments, multiple distance measuring interferometers can be used to monitor multiple degrees of freedom of a measurement object. For example, interferometry systems that include multiple displacement interferometers are used to monitor the location of a plane mirror measurement object in lithography tools. Monitoring the location of a stage mirror relative to two parallel measurement axes provides information about the angular orientation of the stage mirror relative to an axis normal to the plane in which the two measurement axes lie. Such measurements allow a user to monitor the location and orientation of the stage relative to other components of the lithography tool to relatively high accuracy.
Unfortunately, such interferometric stage measurements are sometimes degraded by deformations and/or imperfectations in stage mirrors used to reflect interferometric measurement beams. For example, such mirrors may have local variations in slope that misdirect the measurement beam(s), and which, if not corrected, may be misinterpreted as, e.g., an in-plane rotation of the stage. The stage mirrors are typically elongate mirrors laminated to a wafer or reticle stage. Such mirrors may be characterized before installation in a lithography tool to provide such a correction. However, the installation of the stage mirrors itself may cause additional deformations to them, which would not be characterized.